Electrolytic cells or furnaces for production of aluminum according to the Hall-Heroult method, comprise a generally rectangular, low, flat shell with refractory material and carbon blocks in its sides and bottom. The carbon blocks constitute a vessel for the produced aluminum and for the molten electrolyte. The carbon blocks in the bottom of the vessel are equipped with steel bars for electric coupling of the bus bars for the electric current. The bottom carbon blocks thus form the cathode for the electrolytic cell.
The molten electrolyte, which has a lower density than molten aluminium, consists of molten cryolite, certain inorganic salts, such as for example, aluminum fluoride and calcium fluoride, and dissolved aluminum oxide. Aluminum oxide is consumed during the electrolysis and aluminum oxide therefore has to be added to the electrolyte quite frequently. During operation of the electrolytic cells corrosive fluorine- and sulphur-containing gases are produced.
In electrolytic cells for production of aluminum equipped with self-baking anodes or S.o slashed.derberg anodes, each cell usually is equipped with one substantially rectangular anode. The S.o slashed.derberg anode consists of a permanent outer casing made from cast iron or steel, which casing surrounds the self-baking carbon anode. Unbaked carbonaceous electrode paste is charged at the top of the anode and this unbaked electrode paste is baked into a solid carbon anode due to the heat which evolves during the supply of electric operating current to the anode and the heat from the molten bath. A major feature of the S.o slashed.derberg anode is thus that the baked solid anode moves relatively to the permanent anode casing.
In order to collect gases which evolve during the electrolytic reduction process, S.o slashed.derberg anodes are equipped with so-called gas shirts which run from the anode casing and outwardly and downwardly against the electrolyte where a seal is formed against the crust which forms on the top of the molten electrolyte. The gases which evolve are collected under the gas shirts, sucked off and are burned outside the electrolytic cell. The gas shirts are normally made from east iron which is reasonably resistant against the atmosphere and the temperature in the electrolytic cell. Even if cast iron is reasonably resistant against the gases, the gas shirts have to be replaced at intervals. Cast iron has further a low resistance against the molten electrolyte and by contact with molten electrolyte, for example by splashing, the cast iron erodes very quickly.
Recently, for environmental reasons, it has been proposed to replace the gas shirts with cover plates that run from the anode casing and to the sidewall of the furnace. This solution is disclosed in Norwegian patent no. 1628868. The electrolytic cells are thereby completely closed. The cover plates have been made from steel, but it has been found that even though the distance from the molten electrolyte to the cover plates is substantial longer than the distance from the molten electrolyte to the gas shirts, the steel in the cover plates is eroded rapidly and must therefore be replaced with short intervals.
Further the lower ends of the anode casing made from cast iron or steel is also eroded and must be replaced. The erosion of steel and cast iron parts in the electrolytic cells also gives an increase in the iron content in the produced aluminum.
The CO-containing gas which is produced in electrolytic reduction cells for production of aluminum is collected and combusted by air in burners arranged in gas collection pipes in the cells. These burners which are made from east iron have a short life-time due to erosion and must be replaced frequently.
It has been tried to replace the above mentioned structural parts of electrolytic reduction cells for production of aluminum by other materials such as different kinds of ceramic materials and refractory castables. Thus in Norwegian patent No. 140632 use of a calcium aluminate bonded layered alumina is mentioned as a lining under a steel cover for an electrolytic reduction cell for production of aluminum. In Light Metals, 1992 page 407 to 412 use of a high alumina cement castable is described which shows resistance against molten cryolitt. This castable contains over 90% by weight of fine bauxite. Thus the cement content is very low. Moisture is added in an amount of 3.8-4.0% during mixing of the castable and vibration during casting is essential to promote flowability and maximize density. Thus this cement castable can, due to its low flow, not be used for casting complex shapes. Further there is no indication in the article that the castable is resistant against the gas atmosphere in an electrolytic reduction cell for production of aluminum. Thus cast iron and steel are still the dominant material used for structural parts intended to be in contact with the gas atmosphere in electrolytic reduction cells for production of aluminum.
Thus there is a need for a material which is resistant against the atmosphere that exist in electrolytic cells for production of aluminum and which can be used for the above-mentioned structural parts.